1. Field of the Invention
The present invention relates to radio communication systems, and particularly to a radio communication system that performs relay communication by radio.
2. Description of the Related Art
In radio communication systems, communication is generally performed between a terminal and a radio base station connected to an upper network by a wired link. In recent years, systems have attracted attention in which a relay station that performs relay forwarding by radio is introduced between the radio base station and terminal. Since placing the relay station extends a relay possible area, it is possible to extend the communication area and to improve the communication throughput of the terminal.
FIG. 18 shows an outline of radio relay communication. A relay station 1a and a radio base station 2a are connected by radio, and the relay station 1a and a terminal 3 are connected by radio. The relay station 1a serves as a terminal with respect to the radio base station 2a, and the relay station 1a serves as a radio base station with respect to the terminal 3.
The relay station 1a receives a radio signal sent from the radio base station 2a or the terminal 3 temporarily, performs necessary processing, and sends the signal to the terminal 3 or to the radio base station 2a. Radio relay communication standards include, for example, IEEE 802.16 (WiMAX). In WiMAX, it is expected that the relay communication function will be extended as a mobile multihop relay (MMR).
FIG. 19 shows one type of network in radio relay communication. There are three communication areas A1 to A3. The communication area A1 includes a radio base station 2a-1 and a terminal 3-2, the communication area A2 includes a radio base station 2a-2 and a terminal 3-3, and the communication area A3 includes a terminal 3-1.
Since each unit in the communication areas A1 to A3 share a relay station 1a, a radio communication carrier can extend the communication areas in a step-wise manner while suppressing facility cost by the use of a small number of relay stations.
FIG. 20 shows a state in which connections have been established. When the terminal 3-1 wants to communicate with the radio base stations 2a-1 and 2a-2 through the relay station 1a, the terminal 3-1 establishes a connection C1 with the radio base station 2a-1 to perform communication and establishes a connection C2 with the radio base station 2a-2 to perform communication.
FIG. 21 shows a state in which connections have been established in a network in radio relay communication. A communication area A1 includes a radio base station 2a-1, a communication area A2 includes a radio base station 2a-2, and a communication area A3 includes terminals 3-3 and 3-4. Each unit shares a relay station 1a to perform communication.
In this case, the terminal 3-3 establishes a connection C3 with the radio base station 2a-1 to perform communication, and the terminal 3-4 establishes a connection C4 with the radio base station 2a-2 to perform communication. In either case of FIG. 20 and FIG. 21, the relay station 1a intervenes between the radio base stations and terminals to manage data and control messages sent and received through each connection.
In a conventional radio relay technology, only when a notice indicating that a destination terminal station has not received a message is issued, a radio relay station selectively re-generates the message received from a source terminal station (for example, see paragraph Nos. 0018 to 0033 and FIG. 1 in Japanese Unexamined Patent Application Publication No. Hei-10-215281).
In the above-described conventional networks, where the relay station 1a performs radio relay communication with a plurality of radio base stations, however, the following problems may occur.
When a radio base station sends data to a destination terminal through a connection established, the radio base station performs scheduling to assign a slot (a slot means a position (field) where data or control information is inserted in a frame) in a radio frame, inserts data into the slot, and generates a transmission frame to perform transmission. Each radio base station independently performs such operations for every frame according to a traffic state through a connection managed by the radio base station.
When a plurality of connections are connected to different radio base stations through a single relay station, as shown in FIG. 20 and FIG. 21, data sent from the radio base stations may collide with each other when the relay station receives the data.
FIG. 22 shows a data collision state in the network shown in FIG. 20. When the radio base station 2a-1, which is the destination of the connection C1, and the radio base station 2a-2, which is the destination of the connection C2, assign the whole or a part of transmission data to the same slot in radio frames to be transmitted at the same timing, as a schedule of the transmission data to be sent to the terminal 3-1, the relay station 1a receives both frames, sent from the radio base stations 2a-1 and 2a-2, with the whole or a part of the different transmission data overlapping (with data collision happening), and cannot correctly demodulate the received frames.
The same problem may occur also in the network shown in FIG. 21, where the destinations of the connections C3 and C4 are the different terminals 3-3 and 3-4, because the relay station 1a is shared.
Especially in a WiMAX system, a radio base station performs scheduling to assign a downlink transmission slot sent from the radio base station to a terminal and an uplink transmission slot sent from the terminal to the radio base station.
Therefore, when a relay station, instead of a terminal, performs uplink data transmission to a plurality of radio base stations, scheduling may be performed such that uplink transmission data is assigned to the whole or a part of the same slot in frames to be sent at the same timing from some of the plurality of relay stations. In that case, since the relay station cannot send different data to different radio base stations by using the same slot, the uplink transmission data cannot be sent through each connection at the designated slot.